This invention relates to an apparatus for controlling a continuously variable transmission for use with an automotive vehicle to change the engine brake force when the vehicle is coasting with the accelerator pedal being released.
Some automotive vehicles employ a continuously variable transmission having an input shaft coupled to the engine and an output shaft coupled to the drive shaft for transmitting a drive from the engine to the drive shaft. Such a continuously variable transmission operates with a speed ratio controlled in a manner to bring the speed of rotation of the input shaft into coincidence with a target value calculated as a function of engine throttle position (or accelerator pedal position) and vehicle speed. It is the current practice to decrease the target input shaft speed value as the throttle position decreases. If the vehicle is coasting on a downhill slope, the operator will release the accelerator pedal. This causes the throttle position to decrease so that the target input shaft speed value is changed (decreased) in a direction to weaken the engine brake. As a result, the operator would feel an excessive degree of vehicle acceleration in spite of the fact that the accelerator pedal is released and increase the frequency at which the operator depresses the brake pedal.
For example, Japanese Patent Kokai No. 6-81932 discloses a continuously variable transmission control apparatus intended to reduce the frequency at which the operator depresses the brake pedal when the vehicle is coasting on a downhill slope by setting a great lower limit for the target input shaft speed value to perform aggressive operate engine brake operations. With such a conventional apparatus, however, the operator would feel a sense of incompatibility for the vehicle acceleration caused when the vehicle is coasting on a downhill slope if the vehicle speed comes out of a very narrow range, for example, 20 km/h or less. This is stemmed mainly from the fact that no upper limit has been set for the target input shaft speed.
Furthermore, some continuously variable transmissions are operable in a normal speed change mode (for example, D range) and also in a sport mode (for example, Ds range). In the sport mode, the target input shaft speed value is restricted below a lower limit greater than the limit set for the normal speed change mode to provide an increased engine brake force when the vehicle comes near one of the corners of a downhill slope. It is the current practice to make a change from the normal speed change mode to the sport mode with the use of an inhibitor switch associated with the shift lever. One of problem associated with such a conventional continuously variable transmissions is an engine brake force change against the operator's expectation. For example, the target input shaft speed value decreases to decrease the engine brake force in spite of the fact that the operator changes the speed change mode to the Ds range to obtain an increased engine brake force while the vehicle is coasting with the D range on a downhill slope having a great gradient. This a is stemmed from the fact that the conventional continuously variable transmission control includes no specified relationship between the upper limit set for the target input shaft speed values calculated for the vehicle coasting with the D range and the lower limit set for the target input shaft speed values calculated for the vehicle coasting with the Ds range.